The present invention pertains to induction motors and, more particularly, to an induction servomotor system utilizing a multiphase induction motor having concentrated phase windings.
The use of adaptive control for correcting slip frequency in conventional three-phase induction motors is known in the art. Direct field oriented controllers which require the use of invasive sensing devices, such as Hall effect sensors placed in the magnetic field gap, have generally proven to be impractical. Slip control based on indirect sensing of the magnetic field has also been accomplished. However, because of the distributed nature of the phase windings in a conventional three-phase induction motor, it has proven difficult to separate and accurately sense the stator current components responsible for torque from the magnetizing currents used to generate the field. Furthermore, the magnetic field generated in a conventional three-phase induction motor is sinusoidal in nature making it difficult to sense the peak flux density. The problem is worse at low speeds because the distributed nature of the windings makes it difficult to segregate and sense the relatively lower torque current and voltage values from the dominant IR drop resulting from magnetizing currents in the same phase windings.
Permanent magnet motors are desirable for servomotor applications because of their high bandwidth capability and high efficiency. However, permanent magnet servomotors are substantially more costly to manufacture than induction servomotors. Also, as indicated, slip frequency control in AC induction motors has been difficult to attain.